1. Field of the Invention
This invention relates generally to controlling the flow of fluid by controlling the speed of rotation of a rotating member. More particularly, this invention relates to modulating an airflow within a duct by measuring flow rate and modifying the speed of rotation of a blade.
2. Background Art
One type of conventional valve that is used for air control is, for example, the Phoenix venturi valve, which utilizes a duct section in the form of a venturi. The valve includes a spring or cone that is mounted on a shaft. The shaft is attached to a spring of a constant mass which is designed to maintain a constant airflow regardless of changes in static pressure in the duct. The valve is designed to operate in a pressure independent manner between 0.6 inches and 6.0 inches water column static pressure. The shaft can be modulated to vary the flow while the spring/cone slides on the shaft to maintain its pressure independence. The valve does not directly measure flow, but is calibrated over numerous points and designed to maintain a relatively accurate flow control. The valve can be modulated using either a pneumatic actuator or electric actuator.
Another type of conventional valve is a pneumavalve, such as the Tek-Air Pneumavalve. This valve utilizes a series of bladders that are surrounded by sheet metal and spaced about one inch apart in a metal casing. A 1-10 pound per square inch (PSI) control signal inflates the bladders so that the bladders restrict airflow in a duct. This pneumavalve may be fabricated from either stainless steel or galvanized steel/aluminum depending on the desired application. The pneumavalve is not pressure independent. In order to provide pressure independence, this type of valve must be used in conjunction with an airflow sensor. Since the pneumavalve operates using pneumatic control air, it cannot operate electronically.
VAV (Variable Air Volume) Terminal boxes are devices used to modulate airflow. These devices are typically cylindrical sections of sheet metal with a round damper on a shaft positioned in a duct section. The damper is rotated throughout a 90-degree arc to vary the flow in the duct section. The boxes are not pressure independent and require an airflow sensor. The flow sensor is typically mounted on an inlet portion of the duct section and a controller is used to maintain a desired flow. A pitot tube flow sensor is used to sense airflow and the pitot tube flow sensor has limited turndown flow capacity. Since such dampers are not linear devices, accurate control of airflow is very limited. When the device is moving from a fully closed position to an open position, there is initially a relatively large change in airflow versus control signal and the reverse happens when the device is close to fully open.
It would be advantageous to provide methods and apparatus for the control of fluid flow that do not suffer from the disadvantages of known systems. It would be further advantageous to provide a new type of rotary damper that is efficient, cost effective, reliable and easy to incorporate into both new and existing systems, such as heating, ventilation, and/or air conditioning systems. The present invention provides fluid flow control methods and apparatus having these and other advantages.